An analog-to-digital converter (ADC) is an electronic circuit that converts continuous, i.e., analog, signals to discrete digital numbers. ADCs are in common usage in various fields of electronics and are typically an essential part of most sampled data systems in which they act as a fundamental bridge between the analog and digital domains.
Although ADCs perform substantially the same function, they may do so in different ways. Various types of ADCs exist, their classification being based, at least in some instances, on the manner in which the conversion function is implemented. One way of classifying ADCs is as serial or parallel converters. In parallel converters, the conversion process is completed as a single process within one ‘sample time’ at the conclusion of which the output is produced. Such converters are also called flash converters or direction conversion ADCs. Flash converters typically employ multiple comparators, each comparing the input to a specific reference voltage simultaneously. The parallel output from a flash converter is encoded from the pattern of comparator outputs at each conversion period in pulse code modulation (PCM) form.
In serial converters, the individual bits of the PCM result are computed over several periods of a reference clock typically from the most significant bit (MSB) to the least significant bit (LSB). Traditional successive approximation converters tend to use a comparator to generate a single serial bit at a time, in a sequence from MSB to LSB, which represents the converted PCM value in digital form. The conversion time may be determined by multiplying the total loop settling time for a stable comparison with the number of bits of precision required.
A sub-classification of serial converters is a sigma-delta type ADC, which uses a single comparator but relies on over-sampling and digital processing to compute a more precise result from repetitive samples of the same input. Stigma-delta converters typically over-sample by one or two orders of magnitude and may produce up to 24-bit results.
Other types of ADCs exist as well. There are, for instance, also hybrid ADC implementations. An example of a hybrid ADC is one that uses more than one stage of flash conversion with pipelined sample/hold and comparison circuitry to gain higher conversion rates.
Conventionally, ADCs tend to be monotonic both in conversion linearity and in sample rate. Essentially, a monotonic function is one that preserves the order. For example, a monotonically increasing function is such that, as time increases, so does the function. Similarly, a monotonically decreasing function is such that, as time increases, the function decreases. Monotony in the sample rate describes a constant frequency of conversion.
There is a need for designing converters having improved performance. One desired aspect of such design may include non-monotonic performance. Another desired aspect may include substantially decreasing conversion times.